Electrical contact resistance (ECR, or simply contact resistance) is resistance to the flow of electric current caused by incomplete contact of the surfaces through which the current is flowing, and by films or oxide layers on the contacting surfaces. It occurs at
electrical connections such as switches, connectors, breakers, contacts, and measurement probes. Contact resistance values are typically small (in the microohm to milliohm range).
Contact resistance can cause significant voltage drops and heating in circuits with high current. Because contact resistance adds to the intrinsic resistance of the conductors, it can cause significant measurement errors when exact resistance values are needed.
Contact resistance may vary with temperature. It may also vary with time (most often decreasing) in a process known as
resistance creep.
Electrical contact resistance is also called ''interface resistance'', ''transitional resistance'', or the ''correction term''. ''Parasitic resistance'' is a more general term, of which it is usually assumed that contact resistance is a major component.
William Shockley
William Bradford Shockley ( ; February 13, 1910 – August 12, 1989) was an American solid-state physicist, electrical engineer, and inventor. He was the manager of a research group at Bell Labs that included John Bardeen and Walter Houser Brat ...
introduced the idea of a potential drop on an injection electrode to explain the difference between experimental results and the model of gradual channel approximation.
Measurement methods
Because contact resistance is usually comparatively small, it can be difficult to measure, and
four-terminal measurement gives better results than a simple two-terminal measurement made with an ohmmeter.
* In a two-terminal measurement (as with a typical ohmmeter), the current used to make the measurement is injected through the measurement leads, which causes a potential drop not just across the contact area to be measured but also across the probe contacts and the leads. That means that the contact resistance of the probes and their leads is inseparable from the resistance of the contact area to be measured, with which they are in series.
* In a four-terminal measurement, the current used to make the measurement is injected using a second, separate pair of leads, so the contact resistance of the measurement probes and their leads is not included in the measurement.
Specific contact resistance can be obtained by multiplying by contact area.
Experimental characterization
For experimental characterization, a distinction must be made between contact resistance evaluation in two-electrode systems (for example, diodes) and three-electrode systems (for example, transistors).
In two-electrode systems, specific contact resistivity is experimentally defined as the slope of the
I–V curve at :
:
where
is the current density, or current per area. The units of specific contact resistivity are typically therefore in ohm-square metre, or Ω⋅m
2. When the current is a linear function of the voltage, the device is said to have
ohmic contacts.
Inductive and
capacitive methods could be used in principle to measure an intrinsic
impedance without the complication of contact resistance. In practice,
direct current
Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
methods are more typically used to determine resistance.
The three electrode systems such as transistors require more complicated methods for the contact resistance approximation. The most common approach is the
transmission line model (TLM). Here, the total device resistance
is plotted as a function of the channel length:
:
where
and
are contact and channel resistances, respectively,
is the channel length/width,
is gate insulator capacitance (per unit of area),
is carrier mobility, and
and
are gate-source and drain-source voltages. Therefore, the linear extrapolation of total resistance to the zero channel length provides the contact resistance. The slope of the linear function is related to the channel transconductance and can be used for estimation of the ”contact resistance-free” carrier mobility. The approximations used here (linear potential drop across the channel region, constant contact resistance, ...) lead sometimes to the channel dependent contact resistance.
Beside the TLM it was proposed the gated four-probe measurement
and the modified
time-of-flight method (TOF).
The direct methods able to measure potential drop on the injection electrode directly are the Kelvin probe force microscopy (KFM)
and the electric-field induced second harmonic generation.
In the semiconductor industry, Cross-Bridge Kelvin Resistor(CBKR) structures are the mostly used test structures to characterize metal-semiconductor contacts in the Planar devices of VLSI technology. During the measurement process, force the current (
) between contacts 1 and 2 and measure the potential deference between contacts 3 and 4. The contact resistance
can be then calculated as
.
Mechanisms
For given physical and mechanical material properties, parameters that govern the magnitude of electrical contact resistance (ECR) and its variation at an interface relate primarily to
surface structure and applied load (
Contact mechanics).
Surfaces of metallic contacts generally exhibit an external layer of oxide material and
adsorbed water molecules, which lead to capacitor-type junctions at weakly contacting
asperities
In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (from the Latin ''asper''—"rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, ar ...
and resistor type contacts at strongly contacting asperities, where sufficient pressure is applied for asperities to penetrate the oxide layer, forming metal-to-metal contact patches. If a contact patch is sufficiently small, with dimensions comparable or smaller than the
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
of electrons resistance at the patch can be described by the
Sharvin mechanism, whereby electron transport can be described by
ballistic conduction. Generally, over time, contact patches expand and the contact resistance at an interface relaxes, particularly at weakly contacting surfaces, through current induced welding and dielectric breakdown. This process is known also as resistance creep.
The coupling of
surface chemistry
Surface science is the study of physics, physical and chemistry, chemical phenomena that occur at the interface (chemistry), interface of two phase (matter), phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum int ...
, contact mechanics and charge transport mechanisms needs to be considered in the mechanistic evaluation of ECR phenomena.
Quantum limit
When a conductor has spatial dimensions close to
, where
is
Fermi wavevector of the conducting material,
Ohm's law
Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...
does not hold anymore. These small devices are called
quantum point contacts. Their conductance must be an integer multiple of the value
, where
is the
elementary charge
The elementary charge, usually denoted by , is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single electron, ...
and
is the
Planck constant
The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...
. Quantum point contacts behave more like
waveguides than the classical wires of everyday life and may be described by the
Landauer scattering formalism.
Point-contact
tunneling is an important technique for characterizing
superconductors.
Other forms of contact resistance
Measurements of
thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
are also subject to contact resistance, with particular significance in heat transport through granular media. Similarly, a drop in
hydrostatic pressure
Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...
(analogous to electrical
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
) occurs when
fluid flow
In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...
transitions from one channel to another.
Significance
Bad contacts are the cause of failure or poor performance in a wide variety of electrical devices. For example, corroded
jumper cable clamps can frustrate attempts to start a
vehicle
A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...
that has a low
battery. Dirty or
corroded contacts on a
fuse or its holder can give the false impression that the fuse is blown. A sufficiently high contact resistance can cause substantial
heating in a high current device. Unpredictable or noisy contacts are a major cause of the failure of electrical equipment.
See also
*
Contact cleaner
*
Wetting current
In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curre ...
References
Further reading
* (NB. Free download after registration.)
*
* (NB. A rewrite of the earlier "''Electric Contacts Handbook''".)
*
https://web.archive.org/web/20180520151548/http://www.gbv.de/dms/ilmenau/toc/176968075.PDF -->(NB. A rewrite and translation of the earlier "''Die technische Physik der elektrischen Kontakte''" (1941) in German language, which is available as reprint under .)
* {{cite book , editor-first1=Eduard , editor-last1=Vinaricky , editor-first2=Karl-Heinz , editor-last2=Schröder , editor-first3=Josef , editor-last3=Weiser , editor-first4=Albert , editor-last4=Keil , editor-first5=Wilhelm A. , editor-last5=Merl , editor-first6=Carl-Ludwig , editor-last6=Meyer , display-authors=3 , author-first1=Manfred , author-last1=Huck , author-first2=Eugeniucz , author-last2=Walczuk , author-first3=Isabell , author-last3=Buresch , author-first4=Josef , author-last4=Weiser , author-first5=Lothar , author-last5=Borchert , author-first6=Manfred , author-last6=Faber , author-first7=Willy , author-last7=Bahrs , author-first8=Karl E. , author-last8=Saeger , author-first9=Reinhard , author-last9=Imm , author-first10=Volker , author-last10=Behrens , author-first11=Jochen , author-last11=Heber , author-first12=Hermann , author-last12=Großmann , author-first13=Max , author-last13=Streuli , author-first14=Peter , author-last14=Schuler , author-first15=Helmut , author-last15=Heinzel , author-first16=Ulf , author-last16=Harmsen , author-first17=Imre , author-last17=Györy , author-first18=Joachim , author-last18=Ganz , author-first19=Jochen , author-last19=Horn , author-first20=Franz , author-last20=Kaspar , author-first21=Manfred , author-last21=Lindmayer , author-first22=Frank , author-last22=Berger , author-first23=Guenter , author-last23=Baujan , author-first24=Ralph , author-last24=Kriechel , author-first25=Johann , author-last25=Wolf , author-first26=Günter , author-last26=Schreiner , author-first27=Gerhard , author-last27=Schröther , author-first28=Uwe , author-last28=Maute , author-first29=Hartmut , author-last29=Linnemann , author-first30=Ralph , author-last30=Thar , author-first31=Wolfgang , author-last31=Möller , author-first32=Werner , author-last32=Rieder , author-first33=Jan , author-last33=Kaminski , author-first34=Heinz-Erich , author-last34=Popa , author-first35=Karl-Heinz , author-last35=Schneider , author-first36=Jakob , author-last36=Bolz , author-first37=L. , author-last37=Vermij , author-first38=Ursula , author-last38=Mayer , title=Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren , language=German , edition=3 , location=Berlin / Heidelberg / New York / Tokyo , publisher=
Springer-Verlag
Springer Science+Business Media, commonly known as Springer, is a German multinational publishing company of books, e-books and peer-reviewed journals in science, humanities, technical and medical (STM) publishing.
Originally founded in 1842 in ...
, date=2016 , orig-year=1984 , isbn=978-3-642-45426-4
Materials science
Electrical resistance and conductance